So then what exactly is your question?
Just look at how when everyone was laughing him off for GOP nomination he went and won big.
I'm assuming you realize there is little comparability between the republican nomination battle and the white house battle. As for some of the differences, consider:
1. Electoral math. Trump needs the vast majority of the swing states, many of which have a large number of minorities voting, to win. He polls poorly in many of those states.
2. More people identify as democrat than republican. Trump needs to beat Hillary among independents by a healthy margin. Now that Sanders appears to be supporting Hillary, Trump can count on losing the majority of his supporters (recent polls confirm this). This is in contrast to a month or two ago, when Sanders' supporters appeared more split, as he focused on attacking Hillary. This is a big loss for Trump
3. All major polls have been consistently trending toward Hillary. The result summary isn't as important as the trends in responses to several key questions, such as if you'd even consider voting for Trump vs. Hillary, likeability (which has a strong correlation with results), trust (Hillary's weak area, where she's nearly even with trump), etc. And general election polls are much more accurate than primary polls.
4. Trump is now being vetted like a real candidate. Despite the fact that Bush and co spend a significant amount of money to try and defeat him, he wasn't attacked by the media and press as much (and as effectively) as Trump is currently being attacked.
5. Trump benefited significantly from being the first presumptive nominee, but that advantage is now over
